We are studying the properties and activity of a low molecular weight factor from serum which stimulates the growth of cells in culture. It has been separated from serum by gel chromatography, and purified by paper chromatography. It has an apparent molecular weight of a few hundred and is not a peptide; chemical characterization is under way. The growth factor acts in concert with the high molecular weight components of serum and its mode of action is under investigation. We are also studying the concentration of the growth factor in sera from normal individuals and from cancer patients to determine if there are any differences in the two groups.